Any tips on quickly working out Lowest common multiple,

11 Plus Maths – Preparation and Information

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Silver29
Posts: 151
Joined: Fri Sep 07, 2007 2:12 pm

Any tips on quickly working out Lowest common multiple,

Post by Silver29 »

I was wondering if anyone has any tips/formula for working out Lowest common multiples, highest common factors etc. My daughter gets the right answer most of the time but takes a long while doing it.
Mike
Posts: 625
Joined: Thu Jan 12, 2006 4:29 pm

Post by Mike »

Hi Silver29

The quickest way if the number can not be identified immediately is to use prime factors

i.e 2, 3, 5, 7, 11 etc

Any number can be written as a product of their primes

i.e 6 can be made by multiplying 2 x 3 (both prime numbers)

Work out prime factors as follows:

28
divide by 2
14
divide by 2
7
is a prime number

28 is 2 x 2 x 7

36
divide by 2
18
divide by 2
9
divide by 3
3
is a prime number

36 is 2 x 2 x 3 x 3

Place both sets of numbers closer together

28 = 2 x 2 x 7
36 = 2 x 2 x 3 x 3

The highest common factor is the highest number that goes exactly into both numbers.

The common factors in the example are 2 x 2
Therefore the highest common factor is 4
4 goes into 28, 7 times
4 goes into 36, 9 times

The lowest common multiple is the lowest number that both the numbers will go into.

For lowest common multiple we only count a common value once

28 = 2 x 2 x 7
36 = 2 x 2 x 3 x 3

The lowest common multiple is

2 x 2 x 3 x 3 x 7 = 252

28 goes into 252, 9 times
36 goes into 252, 7 times

Hope this helps

Regards

Mike
Silver29
Posts: 151
Joined: Fri Sep 07, 2007 2:12 pm

Post by Silver29 »

Mike, thanks so much for the step by step explanation - it really makes sense now. :)
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